1. Field of the Invention
The present invention relates generally to ballast controllers, and relates more particularly to ballast control for gas discharge lamps with power factor correction.
2. Description of Related Art
Ballasts have been used for many years as part of lighting systems and gas discharge lamps, and in particular for fluorescent lamps. Fluorescent lamps pose a load control problem to the power supply lines that provide lamp power, because the lamp load is non-linear. Current through the lamp is zero until an applied voltage reaches a starting value, at which point the lamp begins to conduct. As the lamp begins to conduct, the ballast ensures that the current drawn by the lamp does not increase rapidly, thereby preventing damage and other operational problems.
A type of electronic ballast typically provided includes a rectifier to change the alternating current (AC) supplied by a power line to direct current (DC). The output of the rectifier is typically connected to an inverter to change the direct current into a high frequency AC signal, typically in the range of 25-60 kHz. The high frequency inverter output to power the lamp permits the use of inductors with much smaller ratings than would otherwise be possible, and thereby reduces the size and cost of the electronic ballast.
Often, a power factor correction circuit is inserted between the rectifier and the inverter to adjust the power factor of the lamp circuit. Ideally, the load in an AC circuit should be equivalent to pure resistance to obtain the most efficient power delivery for the circuit. The power factor correction circuit is typically a switched circuit that transfers stored energy between storage capacitors and the circuit load. The typical power inverter circuit also employs switching schemes to produce high frequency AC signal output from the low frequency DC input. Switching within the power factor correction circuit and the rectifier circuit can be accomplished with a digital controller.
By controlling the switching in the power inverter circuit, operating parameters of the lamp such as starting, light level regulation and dimming can be reliably controlled. In addition, lamp operating parameters can be observed to provide feedback to the controller for detection of lamp faults and proper operational ranges.
A diagram of a conventional electronic ballast circuit is shown generally as circuit 18 in FIG. 1. A power factor correction (PFC) circuit 20 accepts a line input and provides regulated power to an output stage 22. PFC circuit 20 provides a sinusoidal input current to output stage 22, while also providing a regulated DC bus voltage. Output stage 22 receives the regulated power signal from PFC circuit 20, and provides appropriate control for powering lamp 26. Output stage 22 includes the components and operational ability for preheating, igniting and regulating power to lamp 26.
PFC circuit 20 is typically realized as a boost-type converter that requires a high voltage switch, an inductor, a diode, a high voltage DC bus capacitor and an associated control circuit to produce the desired power signals with the components provided. Output stage 22 is typically realized with a half-bridge driven resonant load to provide appropriate power to lamp 26. Output stage 22 typically requires two high voltage switches, a resonant inductor, a resonant capacitor, a DC-blocking capacitor and an associated control circuit for regulating circuit resonance and power delivery. A block 24 provides a representative diagram of such a conventional control.
In the conventional configuration shown in FIG. 1, a switch Mpfc, a diode Dpfc and an inductor Lpfc are connected in a boost type arrangement. The PFC circuit components Lpfc, Mpfc and Dpfc are operated to charge Cbus during an initial stage, such as during a power-on state. Upon being charged, bus capacitor Cbus supplies power to half-bridge resonant output stage 22 for the remainder of the operation of the circuit. By supplying power to output stage 22, bus capacitor Cbus is rated for high capacitance and high voltage operation, thereby increasing the cost and size of the electronic ballast circuit. In addition, switches M1, M2 are also rated for high voltage operation, and therefore have increased cost and size as well.
A number of faults can occur in the conventional electronic ballast circuit shown in FIG. 1. For example, over-current conditions can occur on the input power line and on the output to lamp 26. In addition, undervoltage conditions can occur on the DC bus. With regard to lamp 26, various faults can occur including failure to strike, physical removal of lamp 26 or when lamp 26 approaches the end of its useful life.
Aside from the above-mentioned faults, the ballast circuit in FIG. 1 can have different operational characteristics based on the tolerances of the components that make up the circuit. Tolerances of the components can also change over time, making it difficult to provide a robust ballast control with good PFC characteristics.
In addition t the above drawbacks, the ballast circuit of FIG. 1 uses 3 control IC's, 21, 23 and 24. Control IC 21 controls switching in PFC stage 20 to correctly modulate the input current to provide good power factor characteristics. Control IC 23 provides overall control of the ballast, including providing control signals to control IC 24. Control IC 24 provides switching signals for the half-bridge composed of M1 and M2 to regulate power delivered to lamp 26. The use of 3 separate control ICs to control the ballast increases the circuit complexity and cost.